1. Field of the Invention
The present invention relates to active power filter technology and more particularly, to a distributed load current sensing system for compensating the harmonic signal and virtual work of each linked load branch.
2. Description of the Related Art
With recent advances in technology, a large number of non-linear load devices are used in inverters, lighting equipment, etc., to enhance the effectiveness and convenience. However, these non-linear load devices also bring a lot of power pollution problems of which harmonic problems are the most severe problems. In order to solve harmonic problems, the conventional measure is to install a power filter in each nonlinear load device.
Regular power filters include two types, namely, the passive power filters and the active power filters. Conventional passive power filters are commonly composed of inductors and capacitors, having the advantages of simple structure and low cost and the disadvantages of large size, heavy weight, being capable of eliminating fixed harmonic only, limited improvement in power factor and being likely to resonate with the power system. Conventional active power filters can effectively inhibit all current harmonics and improve the power factor of the power supply side.
When taking the needs of regular systems into consideration and in order to improve system reliability and to increase system capacity and expandability, active power filters will normally be used. For example, in the load current sensing system shown in FIG. 1, multiple small scale power filters are connected in parallel to the main power trunk.
Many parallel operation of active power filter designs for load current sensing system application are known. These designs can be divided into many groups, i.e., the independent parallel detection method, the load current distribution method, the multiple active power filter-based multi-frequency current compensation method, the multi-frequency switching method and the load information transmission control method. Every power filter control method has its advantages and disadvantages. In the load information transmission control method as shown in FIG. 2, active power filters are respectively installed in respective load branches for compensating different harmonic currents and virtual work. This design can use the power network to transmit information directly, allowing communication with every active power filter so that the active power filters in different load branches can compensate one another to enhance system efficacy. However, distributed active power filters will cause inconvenience in system maintenance and management. Further, the harmonic compensation function of the active power filters can attenuate the message transmission power of the power network. While considering the respective advantages and disadvantages of the aforesaid various active power filter designs, the problems of centralized management and distributed management should also be taken into account.
In a centralized management design as shown in FIG. 3, all active power filters are installed in the power room of the power input terminal and connected in parallel to the main power trunk in front of the load branch to simplify the architecture and management. The installation environment helps prolong the lifespan of the active power filters. In this centralized management design only the main power trunk between power input terminal and the active power filters is maintained clean, and the power network posterior to the active power filters is contaminated with harmonic waves. The centralized active power filters are adapted for detecting the total load current in the main power trunk for the generation of a harmonic compensation signal. However, the total load current can be several amps to several thousands of amps or even up to ten thousands of amps subject to the status of use of the load side power equipment. At this time, a certain degree of detection error can occur due to an excessively large sensing range. The distributed management design shown in FIG. 4 can compensate the drawbacks of the aforesaid centralized management design, keeping the entire power network clean. However, this distributed management design has the drawback of installation environment instability. Further, in this distributed management design, every active power filter can simply control one respective load branch. For enabling the active power filters to support one another, a complicated signal processing and transmitting measure is necessary, leading to management and maintenance inconvenience and shortening the lifespan of the active power filters.